One known family of luminescent materials, some of which are useful as lamp phosphors, are of the general formula: (Ln.sub.w Gd.sub.x Ce.sub.y Tb.sub.z)(M.sub.a Mn.sub.b)(B.sub.c Al.sub.d).sub.5 O.sub.10, where w+x+y+z=a+b=c+d=1, any of w, x, z, b, or d may be zero, Ln is Y or La, and M is Mg, Zn, or Cd. The preparation of various versions has been described in B. Saubat, M. Vlasse, and C. Fouassier, J. Solid State Chem., 34, 271-277 (1980); J. de Hair and J. Kemanade, Paper No. 54, Third International Symposium on the Science and Technology of Light Sources, Toulouse, Apr., 1983; B. Looye, J. de Hair, C. Bakker, U.S. Pat. No. 4,319,161, Mar., 1982; C. Fouassier and B. Saubat, U.S. Pat. No. 4,422,007, Dec. 1983; and elsewhere. The preparations are of two kinds; a solution of the metal nitrates and boric acid is evaporated (or boiled) to dryness, and the resulting solid fired; or the standard method ("shake and bake" ) is used, wherein boric acid and the metal oxides or oxide precursors (hydroxides, carbonates, nitrates, etc.) are intimately mixed as dry powders and the mixture fired. The firing step is sometimes repeated with intervening grinding and remixing.
The standard method can be made to produce a very nearly single-phase material of good brightness and stability in low pressure mercury vapor lamps. However, the particle sizes obtained are typically so large as to require the use of uneconomically large weights of phosphor per lamp. Attempts to grind the material to smaller particle sizes irreversibly degraded it. A method of initially forming the phosphor as small particles has been sought and found.